1. Field of the Invention:
This invention relates to a key telephone indicator control circuit and more particularly to a control circuit for visually indicating the use status of each line in a key telephone system.
2. Description of the Prior Art:
In conventional key telephone systems there are provided tungsten lamps, neon lamps or light emitting diodes as visual indicating elements in each key telephone set in order to indicate the use status of each line or each intercom. The tungsten lamps have the defect of short life, and the neon lamps have defect of generally requiring a high voltage power supply (more than 100 volts for example). Accordingly, long life and low voltage-operated light emitting diodes have recently been used in such indicating circuits.
An exemplary conventional indicating circuit for a key telephone system is shown in FIG. 1, where a contact r1 of a relay is closed by an incoming call signal establishing a circuit through control line L1, light emitting diode LED1, resistor R1 and control line LB to power source E. As a result, the light emitting diode LED1 of each telephone set emits light. When the contact r1 operates intermittently, the light emitting diode LED1 lights or is extinguished in accordance with the period of the intermittent action of the contact r1, and when the contact r1 operates continuously, the light emitting diode LED1 remains lighted. In both cases the use status of line No. 1 is indicated in each telephone set. The current required to operate the light emitting diode is about 10mA. The conventional circuit as shown in FIG. 1 permits current to flow through each light emitting diode Led1-LEDn separately, so that the total capacity of the power source in the key telephone system is the product of the number of lines plus the number of intercom stations and the number of telephone sets and the required current for one light emitting diode. Accordingly, such conventional key telephone systems providing a number of lines and telephone sets create problems with respect to cost and space. Most of the electric power supplied to these conventional key telephone systems is not consumed in the light emitting diodes themselves, but is wasted as joule heat in resistors R1-Rn coupled in series with light emitting diodes LED1-LEDn. As a result thereof electric power efficiency deteriorates on a large scale. Furthermore as large currents flow in control lines LB and L1-Ln, the voltage drop in the cable becomes large and performance of the cable deteriorates due to heat generated therein. Therefore in the prior art, a plurality of thick cables in parallel are required for the control lines respectively. Thus these prior art systems are undesirable and inefficient in view of increased size and high construction costs.